The objective of this research is to identify and characterize the hierarchy of regulatory phenomena which govern the expression of a simple metabolic pathway in a eukaryotic organism. The pathway is that of nitrate assimilation in Neurospora crassa which consists of a pair of pyridine nucleotide-dependent electron transfer enzymes: nitrate reductase and nitrite reductase. Both enzymes of this pathway are adaptively formed in the presence of either nitrate or nitrite, and both are repressed by the end product, ammonia. This latter effect is dominant. Specific immunoelectrophoretic techniques are being employed to determine the kinetics of synthesis and degradation of the nitrate reductase under varying nutritional conditions. In addition, another level of control exerted through a rapid and specific inactivation of the nitrate reductase in vivo is under study, and the elucidation of the mechanism governing this process is a primary goal of this project. Related investigations into the genetic regulation of nitrate assimilation have been initiated to verify previous tentative assignments of known loci as regulatory or structural gene element and to reveal via mutational analysis additional new structural gene and regulatory gene loci. These results will permit research at the molecular biological level to describe the regulatory gene action which manifests the ultimate control over expression of the nitrate assimilatory pathway.